Currently the main method used for spinal vertebra damage is the spine fusion. Fusion is an undesired method of dealing with the problem and can cause damage to adjacent vertebrae since it is the adjacent vertebra which must make up for the angular displacement lost due to the fusion of any two fused vertebrae.
The objective in any spine repair includes maintenance of safety to the spinal cord and its nerve roots and that of supporting the body and head in a variety of postures during normal movement of the arms, legs, and torso. Fusion will not enable the second objective to be fully realize. Coupled with the further damage which can occur in repairing vertebrae with fusion, fusion comes nowhere near accomplishing both objectives.
A wide variety of devices have been proposed for use in implantation into the spinal column. There are several main drawbacks for the devices currently in use. One of the most important shortcomings of the currently used implant devices involves their ability to separate when the spinal column is either placed under tension (as in doing pull ups or inversion therapy) or when loading is lessened. Under either of these conditions, one half of the load device could either be forced from its position in the spinal column, or at best the two load members could fail to re-mate. The result would be the necessity to either re-enter the patient for adjustment or an exterior attempt to get the load members to re-mate.
Other prostheses have attempted to imitate the intervertebral fribrocartilaginous discs. Bonding and support which imitates the human action of such discs is not realistically achievable. Again, since these soft discs can always slip out of their place, a disastrous breakdown of such a system is always possible.
It is clear that there is a severe need for a superior surgical joint replacement device; particularly, a replacement for a dislocated, ruptured or damaged vertebra due to spondylolysis. An estimated annual incidence of spinal cord injury (SCI), and its associate vertebrae injuries, is between 30 and 40 cases per million population in the U.S. Based on the 1992 census population of 254 million, these rates correspond to 7,600 and 10,000 new cases each year.
The number of people in these United States who are alive today and have SCI has been estimated to be between 721 and 906 per million population. This corresponds to between 183,000 and 203,000 persons. Spinal fusion is frequently used as a treatment for lower back pain and intervertebral disk degeneration, and the use of internal fixation has increased the ability of a surgeon to obtain a solid fusion. There is increased concern, however, that the biomechanical rigidity of the fusion and internal fixation may predispose adjacent spinal motion segments to rapid deterioration. Long-term follow up of patients undergoing a successful fusion indicates that 50 percent will continue to have complaints of back pain. As in other joints, alternatives to fusion of a spinal motion segment have inherent advantages.
Researchers have attempted to design a successful intervertebral disk arthroplasty device for years. For example, U.S. Pat. Nos. 4,946,378 & 4,874,389 discloses an artificial disk having a pair of end bodies with a medical synthetic polymeric intermediate member held between the end bodies. The intermediate member apparently provides some flexibility. Somewhat similarly, U.S. Pat. Nos. 4,932,975 & 5,002,576 discloses an artificial disk having end cover plates separated by a closed corrugated tube which is filled with a visco-elastic material, like a body-compatible silicone or elastomer. Other approaches are shown in U.S. Pat. Nos. 4,349,921, 4,714,469, 4,759,769, 4,863,476, 4,936,848, 4,997,432, 5,047,055, 5,071,437, 5,306,308, 4,349,921 and PCT patent WO 92/14423 discloses an artificial disk having convex superior and inferior surfaces corresponding to the adjacent vertebrae surfaces and being formed from two components to allow flexion and extension between the two components. U.S. Pat. No. 4,759,769 discloses and artificial disk having upper and lower members hinged together at the rear portion and biased apart at the front portion by stiff coil springs. U.S. Pat. No. 4,863,476 discloses a two portion spinal implant that is expandable so as to increase the spacing between the adjacent vertebrae. U.S. Pat. No. 4,936,848 shows an artificial disk having a spherical shape that is hollow and rigid. The sphere wall contains fenestration, open to the sphere cavity, for placing bone fragments therein. U.S. Pat. No. 4,997,432 shows an artificial disk having plates separated by a sliding core body normally consisting of a synthetic material. U.S. Pat. No. 5,047,055 discloses an artificial disk made of hydrogel material having a specified compressive strength and, when hydrated having the shape of a human disk. U.S. Pat. No. 5,071,437 shows an artificial disk having two rigid end-plates separated by, and connected to an elastomeric core material having flexure properties similar to those of a human disk. Finally, U.S. Pat. Nos. 4,595,663, 5,037,438, 5,425,773 and Re, 32,449 discloses the use of ceramic material, including Zirconia, for applications such a joint replacement. There are certain basic criteria a successful intervertebral disk arthroplasty device must fulfill.
Fatigue strength of the materials used in any joint replacement is of utmost importance. Since the average age of patients undergoing fusion is 36 years old, the life span of the device should exceed 51 years. Assuming the average patient walks 2 miles per day, with a stride of 2 feet=5,280 strides per day * 365 days=1,927,200 strides per year * 51 years=98,287,200 cycles+400,000 significant bends in the spine. A conservative estimate of the number of spinal loading cycles over 51 year period would be 98,687,200 cycles. To provide a safety factor, round off at 100 million cycles. A device for implant into the Human spine should be designed to have a fatigue limit of 100 million cycles. In addition to such durability, the material for a successful intervertebral disk arthroplasty device must be biocompatible. The wear of the implant must be kept to a minimum. Although the implant should be small enough to be contained within the anatomic confines of normal disk space, it is recognized that it may be advantageous to increase the prosthetic disk height in order to over distract the disk space to unload the joints posteriorly.
The present invention satisfies all these criteria.